Method of applying protective coatings to metal articles

ABSTRACT

A coating for metal articles which is resistant to chemical and mechanical attack is produced by first depositing a highly homogeneous, finely crystalline zinc layer on the base article; then producing a chromate layer on the zinc deposit by submerging the article in a chromic acid solution with a PH value of less than 2, preferably around 1.5; then, after rinsing the remnants of the chromic acid solution from the article, coating the rinse wet article in a dispersion of synthetic material and in the presence of a reducing agent, with synthetic materials of nonwaxlike character; and then subjecting the base article with the synthetic coating thereon to a thermal treatment in order to reduce the hexavalent chromium of the chromate layer to trivalent chromium and to harden the synthetic coating by drying.

United States Patent [191 Labenski et a1.

METHOD OF APPLYING PROTECTIVE COATINGS TO METAL ARTICLES Inventors: Wolfgang Labenski, Leimen; Heinz Paul Schapitz, Walldorf; Hans-Peter Wessel, Heidelberg, all of Germany Assignee: Mecano-Beindy GmbI-I, Heidelberg,

Germany Filed: Sept. 10, 1971 Appl. No.: 179,524

Foreign Application Priority Data Sept. 21, 1970 Gennany 2046449 US. Cl. 148/6.2, 117/75 M, 204/55 R Int. Cl. C23f 7/26 Field of Search 148/6.2; 204/55 R References Cited UNITED STATES PATENTS [451 Apr. 30, 1974 5/1966 Kennedy 204/55 R x 6/1965 Schiffman l48/6.2

Primary Examiner-Ralph S. Kendall [57] ABSTRACT A coating for metal articles which is resistant to chemical and mechanical attack is produced by first depositing a highly homogeneous, finely crystalline zinc layer on the base article; then producing a chromate layer on the zinc deposit by submerging the article in a chromic acid solution with a Pl-l value of less than 2, preferably around 1.5; then, after rinsing the remnants of the chromic acid solution from the article, coating the rinse wet article in a dispersion of synthetic material and in the presence of a reducing agent, with synthetic materials of non-waxlike character; and then subjecting the base article with the synthetic coating thereon to a thermal treatment'in order to reduce the hexavalent chromium of the chromate layer to trivalent chromium and to harden the synthetic coating by drying.

9 Claims, No Drawings 1 METHOD OF APPLYING PROTECTIVE COATINGS TO METAL ARTICLES BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates generally to a method of providing metal articles with a protective coating which is resistant to chemical attack as well as to mechanical impairment. More specifically, the invention is concerned with an improved method of applying a protective coating to a steel base body and which involves the galvanic depositing of a zinc layer on the base body and the subsequent chemical application of a chromate coating to the zinc layer.

2. Description of the Prior Art In coating methods of the mentioned character the chromate coating serves to passivate the surface of the zinc layer. By chromatizing, the surface of the zinc layer is provided with a protective coating which consists mainly of hexavalent and trivalent zinc chromate. The chromatizing is always effected chemically in an aqueous solution as disclosed, for instance, in German DAS No. 1,167,621 applied for'May 14, 1959 by A.

Raymond.

German DAS No. 1,219,765 applied for Oct. 7, 1961 by AMCHEM S. A., further discloses a method of passivating metal surfaces, wherein the passivating is effected solely by chromium oxide and which results in a product of improved capacity to resist atmospheric influences. As a rule, such ambient influences are to be seen in the S content of the air within the vicinity of industrial areas or in the corrosive attack by spray water and salts. Under those conditions the mentioned chromating is usually insufficient so that it becomes often necessary to apply a supplemental protective covering with a commercial type of bake on lacquer.

In order to produce a protective coating of improved flexibility and also one that is more corrosion and impact proof, a method of passivating metal surfaces has further been disclosed in German DAS No. 1,250,233 applied for Sept. 17, 1963 by Amchem Products, lnc., wherein a polymeric resin compound is added 'to the acid, aqueous solution containing reduced chromium,

which is utilized for the depositing of a chromium oxide layer. The compound is subsequently desicated so that a flexible upper protective layer is produced.

Finally, German DAS No. 1,246,357 applied for June 18, 1958 by Heintz Manufacturing Company, discloses a method of producing thin coatings, wherein surfaces of iron, copper, aluminum, zinc and other metals are provided with a protective coating in a dispersion of chromic acid or water soluble dichromates of at least bivalent metals with resins of nonwaxlike character.

In any given case the choice of a suitable method is determined by the purpose for which the metalarticle to be coated is intended.

SUMMARY OF THE INVENTION The invention is summarized in the foregoing Abstract of the Disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Although the disclosure hereof is detailed and exact to enable those skilled in the art to practice the invention, the physical embodiments herein disclosed merely exemplify the invention which may be embodied in other specificstructure. The scope of the invention is defined in the claims appended hereto.

In the automotive industry the requirements for corrosion resistance of the parts which are vital for the safety of the vehicle, as for instance the corrosion resistance of the brake lines, have become increasingly exacting, and in conjunction therewith also the regulations which have been promulgated for the quality proof at all stages of the manufacture of such products. With these considerations in mind, the invention contemplates to provide a corrosion protection which not only satisfies the existing requirements for the finished product but which, moreover, will stand up under the loads to which the coated product becomes subjected during final manufacture and subsequent utilization, and which protection will be particularly adapted to take care of these problems. Finally, it must be kept in mind that in the automotive industry the quantity demands for needed parts, as for instance brake lines, are very great, so that a method must be found which in practice has a very high output capacity.

According to the present invention it is proposed that the zinc coating be formed as a finely crystalline, highly homogeneous zinc layer, that after rinsing of the remnants of the chromic acid solution from the article, the rinse wet article be coated while in a dispersion of synthetic materials of a non-waxlike character, and that the base article with the synthetic coating thereon be subjected to a thermal treatment in order to reduce hexavalent chromium of the chromate layer to trivalent chromium and to harden the coating of synthetic materials by drying.

Practicing the invention achieves the desired object of producing a multi-ply protective layer which satisfies the existing requirements. The article protected in that manner may be distorted within wide limits without adverse effect upon the protective coating. This advantageous feature is due, among other things, to the intimate bond between the chromate layer and the synthetic material, which is obtained by the fact that the high activity of the chromic acid solution, assisted by the low PH value of the chromic acid solution, produces a relatively rough and, immediately upon its formation, spongy chromate layer which in conjunction with the subsequent procedural steps establishes the intimate contact. What takes place is an optimal diffusion of the synthetic material dispersion into the porelike hollow spaces of the still spongy chromate layer, and also an exchange of the water contained in said spaces, with the synthetic material dispersion, such exchange being favored by the immersion of the still wet, just chromatized article into the synthetic material dispersion. During the final thermal treatment, this deep reaching intimate interlock solidifies and an interface bonding layer forms which prevents the well known brittleness of the chromate layer from becoming disadvantageously effective.

The finely crystalline zinc deposit is preferably effected in sulfuric acid electrolyte and by means of extremely high current densities, as between 40 and Ampere/dm the electrolyte temperature being kept at 50 to 55 C. and at a high rate of flow, and additives on an organic basis being used to produce a finely crystalline structure of the zinc deposit.

The additives on an organic basis which function as fine grain and luster producers, ensure the preservation of the fine structure of the zinc layer in spite of operation with high current densities, so that a high output capacity can be obtained. The presence of organic substances in the electrolyte is to a certain extent responsible for an implantation thereof into the zinc layer where it produces an inhibiting effect. The thermal treatment should be performed at temperatures of at least 100 C. in order to reduce a large portion of the hexavalent chromium to trivalent chromium. Such treatment, moreover, provides for rapid dessication and hardening of the synthetic layer. The generally known upper temperature limits required by the synthetic material must of course be observed during the thermal treatment of the synthetic layer. The synthetic layer may be reinforced by applying one or more additional layers of synthetic material thereto after the thermal treatment.

High product outputs may be obtained by a further development of the method whereby the individual method steps are performed progressively in continuous succession. Such operation which provides a constant speed of the base body during its progress through the individual method steps is inventively significant because it affords the best way of observing, for instance, the inventive condition that the just chromatized body be introduced while still wet into the synthetic material dispersion. Moreover, continuous operation in the mentioned manner insures a uniform quality over a prolonged period of time.

A further way of perfecting the method involves the use of a base body which has been provided with a thin copper coating. Such coating may, for special uses of vention passes through transfer routes along automatic severing devices, bending equipment and apparatus for swaging or flanging the ends of tubing sections provide them with seats for pipe couplings and for connections to circuit components.

During bending, for instance, very small bending radii may be incurred in the order of three times the tubing diameter, and such bending radii are accommodated by tubing processed in accordance with the invention, without damage to the protective coating because the outer synthetic layer when coacting with a bending tool serves as an intermediate and slide layer and prevents rubbing damage of the vulnerable metal surfaces. The service conditions to which the finished product is exposed when installed in a motor vehicle, are commonly known.

In order to check the corrosion resistance of the tinished product a multitude of test procedures have been worked out, the following of which have become part of the conditions for delivery:

a. sweat water test (Tropic test) according to DIN 50017;

b. testing an aggravated industrial atmosphere (Kes- 5 ternich test) according to DIN 50018/2, O S;

BRAKE LINE TESTS Preparation Sweat water Test Kesternich Test as Salt Spray Test as of as per DlN 50 017 per DIN 50 018/2. OS per ASTM-B H7 No. Test Specimen White Grate Red Grate White Grate Red Grate White Grate Red Grate l Sawed l. round not tested 1. round 10 I2 24 h 240-3l2 h.

g V Straightener! (100%) round Bent 2 Sawed straightened 30. round not tested 8. round 25 26 360 h. 1032-] I04 h Swagpd (20%) round (at the ends) the ends) 3 Sawed straightened 30. round not tested 15. round 29 216 h 744-8l6 h Bent (-60 round (at the bend) the bend) 4 Sawed Straightened 30. round not tested [3. round 28 35 528 h l032-l200 h Swaged (l030% round (at the ends) (at the ends) NOTE: In each case l0 specimens were tested, the results of which all lie within the indicated scatter range.

The specimen N0. 2 had a single synthetic coating; the specimens No. 3 and No. 4 had a double synthetic coating.

. stage. Frequently, for instance, the tubing which has been covered with a protective coating according to the invention, is put up in bunches of pieces cut to length, stored and transported when needed. During such handling the synthetic layer, due to its elasticity, prevents mutual damaging of the surfaces. In the production of brake lines for motor vehicles, for instance, the tubing which has been coated according to them- A further development of the method which lends itself to the production of tubing for the automotive industry is distinguished in that the tubing lengths to be processed are connected water tight together in a manner known per se so that they will present a continuous tubing line before their surfaces to be zinc coated are cleansed preparatory to the zinc coating. This expedient makes it possible to operate for a long period of time with an article resembling an endless length of tubing.

For the performance of a continuous process according to the invention a suitable apparatus may be provided incorporating devices aligned in series to prepare the surface to be zinc coated for the zinc deposit; and to effect the chromatizing, the synthetic coating and the thermal treatment.

The invention will be further explained hereinbelow by examples, the processing of tubings by the continuous method having been selected for exemplification.

The pieces of tubing which have been connected together continuously end to end, first pass through preparatory treatment stages, that is, for electrolytic grease removal which is preferably accomplished by means of a method which avoids an undue hydrogen diffusion into the tubing. Prior to the insertion into the zinc baths the decapping is effected. Sulfuric acid deposition of zinc as employed for the example involves three consecutively arranged zinc baths which, although identical in their chemical composition are nevertheless separately constructed in order to afford the possibility of distributing the applied high current densities over several current rollers. Sulfuric acid zinc baths have been chosen because these are superior to chloride andfluoro borate electrolytes in that their manipulation in the bath structure is relatively simple. Moreover, the sulfuric acid electrolyte is not nearly so aggressive. An additional fact is that the precipitates of these baths are more resistant. The corrosion character of zinc coatings becomes, as such, very problematical if high current densities are employed. Generally, an acceptable corrosion protection can still be obtained with current densities of around 40 Ampereldm However, if current densities are employed which lie at 70 to 100 Ampere/dm a crystalline structure of such coarseness is obtained. that there can no longer be any mention of a homogeneous deposit. This phenomenon which in itself should deter a man skilled in the art from the introduction of high current densities can be avoided by -a further expedient, namely, by the adding of organic substances.

An example for the composition of the zinc bath is given as follows:

170 g i 5 g zinc 1.5 g i 0.3 g aluminum PH value: 3.3;

Electrolyte temperature 6 g i l g boric acid 50-55 C.',

Current density 40400 Ampere/dm 3.3 g t 0.3 g zinc chloride The high current densities, moreover, require a strong flow of the electrolyte. Naturally, a supplemental cathode agitation is also necessary which, however, is inherent in successive processing installations because the tubing moves through the bath. in continuously operating plants continuous filtering of the electrolyte is also necessary in order to insure a uniform quality.

The zinc coated tubing runs subsequently into the chromatizing device. Forthe chromatizing a chromic acid solution is employed which has a PH value of less than 2, preferably 1.5, and which contains chlorine ions and the like-in such proportion that per mol chromic acid less than 1 mol chlorine ions will be present.

Subsequently, the synthetic coating takes place in a dispersion for which the following synthetic materials may be used: epoxy resins, phenol resins, styrole, acrylic resins and others.

We claim:

1. The method of providing a metal body with a protective coating which is resistant to chemical and mechanical attack, which comprises the steps of galvanically coating the metal body with zinc of a finely crystalline, highly homogeneous structure; applying a spongy chromate coating over the zinc coating by immersing the metal body in a chromic acid solution with a PH value of less than 2; rinsing remnants of said chromic acid solution from the metal body; applying a coating containing a synthetic resin dispersion and a reducing agent over the spongy chromate coating while still wet;

and thereafter subjecting the metal body to a thermal treatment to convert the chromate coating to trivalent chromium and enable the synthetic resin to permeate the spongy chromate coating and solidify and interlock therewith.

2. The method set forth in claim 1, wherein application of said zinc coating is effected in a sulfuric acid electrolyte under extremely high current densities between 40 and Ampere/dm said electrolyte being kept at elevated temperatures in the range from 50 to 55 C., and at a high flow rate; and organic compounds being included in the electrolyte and effective to produce said finely crystalline structure of said zinc coatmg.

3. The method set forth in claim 1, wherein chlorine ions are contained in said chromic acid solution and in such proportion that per mol chromic acid less than one mol chlorine ions will be present.

4.The method set forth in claim 1, wherein said thermal treatmentis effected at a temperature of at least 100 C.

5. The method set forth in claim 1 and further including the step of applying one or more additional synthetic coatings after said thermal treatment.

6. The method set forth in claim 1, wherein a preparatory treatment, said zinc coating, said chromatizing, said synthetic coating and said thermal treatment are applied to said base body in continuous succession.

7. The method set forth in claim 1, wherein a body provided with a thin copper coating is utilized as said base body.

8. The method set forth in claim 1 wherein the method body comprises small diameter tubing for exposed fluid lines of automotive vehicles.

9. The method set forth in claim 8, wherein tubing lengths to be processed are connected together to form a continuous tubing line before their surfaces to be zinc coated are cleansed preparatory to said zinc coating. 

2. The method set forth in claim 1, wherein application of said zinc coating is effected in a sulfuric acid electrolyte under extremely high current densities between 40 and 100 Ampere/dm2, said electrolyte being kept at elevated temperatures in the range from 50* to 55* C., and at a high flow rate; and organic compounds being included in the electrolyte and effective to produce said finely crystalline structure of said zinc coating.
 3. The method set forth in claim 1, wherein chlorine ions are contained in said chromic acid solution and in such proportion that per mol chromic acid less than one mol chlorine ions will be present.
 4. The method set forth in claim 1, wherein said thermal treatment is effected at a temperature of at least 100* C.
 5. The method set forth in claim 1 and further including the step of applying one or more additional synthetic coatings after said thermal treatment.
 6. The method set forth in claim 1, wherein a preparatory treatment, said zinc coating, said chromatizing, said synthetic coating and said thermal treatment are applied to said base body in continuous succession.
 7. The method set forth in claim 1, wherein a body provided with a thin copper coating is utilized as said base body.
 8. The method set forth in claim 1 wherein the method body comprises small diameter tubing for exposed fluid lines of automotive vehicles.
 9. The method set forth in claim 8, wherein tubing lengths to be processed are connected together to Form a continuous tubing line before their surfaces to be zinc coated are cleansed preparatory to said zinc coating. 